1. Field of the Invention
The present invention relates to a liquid crystal display device, and is suitable for a liquid crystal display device which includes aback illumination device (a backlight) which arranges solid light emitting elements such as light emitting diode elements as an illumination light source of a liquid crystal display panel on a side surface of a light guide plate.
2. Description of the Related Art
In a miniaturized information terminal such as a mobile telephone or a portable information terminal, as a display device thereof, a liquid crystal display device which is electric power saving, small-sized and light-weighted has been popularly used. With respect to the liquid crystal display device, there have been known a liquid crystal display device which uses an external light as an illumination means for visualizing an electronic latent image which is formed on a liquid crystal display panel or a liquid crystal display device which installs an auxiliary illumination device (hereinafter simply referred to as an illumination device) on a back surface or a front surface of the liquid crystal display panel. The illumination device which is mounted on the back surface of the liquid crystal display panel is usually referred to as the backlight, while the illumination device which is mounted on the front surface is generally referred to as a front light.
As the backlight of the miniaturized information terminal such as the mobile phone or the like, there exists a backlight which includes a light guide plate which arranges a cold cathode fluorescent lamp on a sidewall (a side edge, an incident surface) as in the case of a notebook personal computer having a relatively large sized display screen. However, it is preferable to use a backlight which includes one or a plurality of solid light emitting elements which are represented by light emitting diode elements which exhibit the small power consumption in place of the cold cathode fluorescent lamp.
With respect to a backlight of a type in which a plurality of light emitting diode elements are used as a light source, light which light emitting diode elements emit is incident on a side surface (incident surface) of a light guide plate, and the light is irradiated from an upper surface of the light guide plate as a surface light source, there exists the large brightness difference between a portion in front of the light emitting diode elements and a portion between the light emitting diode elements in the inside of the light guide plate. Particularly, this tendency is apparent in the backlight which arranges a prism sheet having downward prism grooves on an upper surface of the light guide plate (between the light emitting diode element and the liquid crystal display panel). Although various proposals have been made to decrease the brightness difference among the light emitting diode elements when a plurality of light emitting diode elements are used, these proposals are less than optimal.
As a backlight of this type, trials for overcoming the brightness irregularities by focusing on a shape of grooves formed in a light guide plate have been proposed in a U.S. Patent Pub. No. 2004/0120139A1 (patent literature 1), U.S. Pat. No. 6,921,178B2 (patent literature 2), and Japanese Patent Laid-open No. 227956/2004 (patent literature 3).